This invention relates generally to a method and apparatus for providing panic protection for a circuit and its load. This invention has particular application in the protection of power converter circuits used to control electric lamps.
Florescent and High Intensity Discharge (HID) lamps are commonly controlled by an electronic ballast. The ballast drives the lamp with an alternating wave of a particular frequency. The reason this is done is that due to the physics of such lamps, current cannot pass in one single direction continually without adversely effecting the operation of, or damaging, the lamp. As well, due to the physics and composition of the electronic ballast which drives the lamp, there are inherent limits as to the voltage which can exist across, and the current which can flow through, the ballast and the lamp. Voltages in excess of the maximum rated voltage for the lamp, as well as currents in excess of the maximum rated current for the lamp, will damage the lamp. These ratings vary with the type and robustness of the lamp in question. Additionally, and more of a risk factor, is the ballast itself. Ballasts commonly contain a resonant capacitance in parallel with the lamp. If an over-voltage condition across a lamp occurs, i.e., the lamp voltage exceeds the ignition voltage plus a margin, this resonant capacitance will be destroyed. Accordingly, protections for an over-voltage or an over-current situation are very important in the design of lighting systems. Such situations are known as fault conditions. As well, panic protection (i.e., protecting against a transient surge with extremely fast onset) and the related ability of the lamp driving circuit to automatically shut itself off, or dramatically decrease the voltage across the lamp, and the current running through it, is necessary. The existence of protections such as these in the ballast system can potentially save the lamp and ballast, as well as peripheral equipment.
On the other hand, some apparently fault or panic conditions are merely noise, and are so transient so as to present no reason to modify the AC waveform sent to, or the power it delivers to, the lamp. That is, there is no enduring organic problem with either the circuit, or the lamp load driven by the circuit, or any associated components or elements, to mandate changing the driving signals of the lamp, and thus visibly diminishing the performance of the lamp.
There are a variety of methods to sense a fault condition. Sometimes, electronic ballast overload protection is effected using analog comparators, where an overload protection circuit comprising analog comparators is hardwired to the lamp, and designed to continually sense the lamp voltage and the lamp current. If the value of the voltage or current is larger than a reference value built into the comparator, the comparator will output a signal to shut down the switching pulse generated by the ballast and the lamp will not be driven. Many ballasts are microprocessor based. These microprocessor-based ballasts may also use analog comparators to detect the lamp voltage or the lamp current and shut down the switching signal when there is an overage. Alternatively, the output of the analog comparator can be sent to the central processing unit (CPU) of the microprocessor driving the lamp with a pulse width modulated (PWM) signal. The comparator output will activate a software program that will change the PWM module""s settings to either shut down the switching pulse, change the frequency, or reduce the pulse switch so as to insure the power delivered to the lamp will be low enough to resolve the fault condition.
A recent proposal, disclosed in U.S. Pat. No. 5,696,431, commonly assigned with the instant application, which is incorporated herein by reference. This patent discloses, upon sensing of an overload condition, immediately increasing the switching frequency to its maximum setting (and thus the switching period to its minimum) for the duration of the fault condition.
Another, similar, solution is disclosed in a commonly assigned pending U.S. patent application of Shenghong Wang, entitled xe2x80x9cMETHOD AND APPARATUS FOR PROVIDING OVERLOAD PROTECTION FOR A CIRCUITxe2x80x9d, also incorporated herein by reference. In this latter application, in describing an exemplary fixed-frequency, pulse-width-controlled system, dedicated digital hardware is employed to set the pulse width to a minimum value upon the sensing of an overload condition. Software is programmed to return to the normal mode of operation many switching cycles later.
There are problems inherent in the methods currently used to provide overload protection for an electronic ballast. What will first be discussed is the pure hardware solution using analog comparators. The use of analog comparators for ballast failure protection is both unstable and unreliable. In the first instance, the parameters of the protection circuit are sensitive to variations in temperature and process technology, and are therefore plagued by substantial variability from the nominal values of maximum current and maximum voltage that they protect against. Additionally, even assuming that the reference voltages and reference currents in the comparators can be suitably or acceptably calibrated for the circuit in which they are used, the resulting protection circuit would not be programmable or of much use in any other circuit. Accordingly, in the analog pure-hardware protection circuit, it would only be useful for protecting against one particular voltage and one particular current limit. Such a protection circuit would neither be universal or adjustable in any sense and could not be used as a standard component of a ballast designed to control a variety of lamps and lamp driving circuits.
On the other hand, using a pure software solution does gain the advantage of flexibility, in that the maximum current and maximum voltage against which the protection circuit protects can be programmable. Thus the circuit can be used with a variety of lamps and lamp driving circuits, as well as offering flexibility to change the overload maxima as noise and other conditions may warrant. All that is needed is to reprogram the CPU to change the PWM signal upon a particular condition (e.g., maximum voltage) occurring for example, if the override circuit trips at too low a voltage, then values utilized by the software can simply be changed. The hardware solution would require various component changes.
The speed with which the overage decision-making process can be made in the pure-software solution is generally too slow to protect circuits from a panic or near panic situation. A panic situation is one where there is a severe current overage or a severe voltage overage running in the lamp and the protection circuit must respond immediately if the damage to the ballast is to be prevented. Immediately in such a sense means within one switching cycle of the lamp. The CPU and software simply take too long to respond.
Finally, even in those systems using a digital hardware solution, the performance of the lamp is diminished for the duration of the response to the fault condition. Thus, in all instances, the driving signal to the circuit load, which in a lighting circuit is a lamp, is shut down or modified in response to each and every sensed fault condition. This is superfluous, and a needless interruption of service. In each of the prior art solutions, it takes several switching cycles to return to the normal mode of operation. However, many fault conditions are not xe2x80x9crealxe2x80x9d, in the sense that they indicate an enduring problem with the circuit or the driven load. These fault conditions can be the result of noise, or some other cause unconnected to the circuit or its components and associated devices. Such fault conditions tend to be of very short duration, and often resolve themselves. In effect, reacting to each and every sensed fault condition with the full battery of system remedies is akin to administering a complex treatment to every patient registering a false positive on a somewhat insensitive medical test.
In general, providing overload protection via analog hard wiring is inaccurate and inflexible. Providing the protection in the CPU software means that any overload condition has to be processed through the CPU, because an adjustment must be made, and a new PWM signal generated. This takes too long. Providing it via dedicated hardware has additional costs and complexity, and the modified pulse train to the lamp is. In any case, a fault condition of any cause or duration triggers the fault response and affects performance. Where such fault condition is only transient or temporary, and is not indicative of any enduring and potentially dangerous problem, there is no reason to modify the performance of the lamp, and thus decrease the service provided by the lighting system.
As a result of the foregoing discussion, clearly a significant need exists in the art for an overload protection circuit for an electric lamp and ballast which can provide both the programmable flexibility of a software solution with the immediacy and speed that can only be currently delivered by a hardwired overload protection circuit. There is further a need to distinguish transient fault conditions from enduring ones that mandate modifying the waveform delivered to the load. Such distinction would protect the circuit and its components, yet at the same time not needlessly modify the driving signal to the load until it can be determined that the fault condition is in fact real.
The above-described shortfalls of the prior art are overcome in accordance with the teachings of the present invention which relates to an apparatus and method for providing overload protection for a circuit by means of a hybrid software and hardware solution. In a preferred embodiment, this circuit is used in a digital ballast controlling electric lighting using a half-bridge power converter, such as is commonly used in the control and operation of gas discharge lamps. The invention can easily be expanded to other power conversion circuits driving a wide variety of loads.
In a preferred embodiment, upon the sensing of a fault condition the circuit reacts immediately to temporarily block all driving signals from reaching the load. At the same time, the signals themselves remain intact, and are neither modified nor terminated until it can be confirmed that the fault is a xe2x80x9crealxe2x80x9d one.
Effectively, the driving pulse trains are overridden, or blocked, via hardware logic gates during initial sensing of overload conditions for a user defined short time interval. If such conditions persist beyond a user defined number of such blockings, the pulse trains can then be modified, attenuated, or terminated, as defined and set by the user, via software.